The invention is related to a method for performing a heat treatment on steel rings, in particular for bearings, whereby the outer surface and the inner surface of a preheated ring are exposed to different flows of a cooling medium such that the regions of the ring adjacent those surfaces are subjected to different cooling processes.
A method of this kind is known from laid-open Japanese patent application (SHO) 61-235509. By means of this method, a bearing ring can be manufactured the inner and outer surfaces of which have different hardnesses. For this purpose, a steel ring is partially quench-hardened by subjecting one surface thereof to a coolant spray. At the same time, the other surface is subjected to a flow of compressed air. Thereby, said outer surface obtains a hardness corresponding to the quenching process, whereas the inner surface is allowed to obtain a hardness corresponding to a tempering process.
Although this prior art method offers some improvement in the mechanical properties of bearing rings, its useful application is limited. In particular, said process is intended to offer improvements in bearings to be used in a mast roller of a fork lift device, wherein the outer surface of the outer ring is in rolling contact with a mast rail.
The purpose of the invention is to offer a general heat treatment method for producing rings, in particular for bearings of any type, which have a much broader application. In the normal process of manufacturing such bearing rings, in a first step a ring is cut from a tube or forged from a suitable steel material. According to a further possibility, such ring may be formed by a cold rolling and hot forming process. Subsequently, said rings are subjected to a turning operation, and then they are heat treated.
The heat treatment entails the heating of the rings to a temperature of about the austenitizing temperature, then performing a quenching operation in order to obtain a martensite structure, and finally tempering.
In this process, the ring in question becomes distorted as a result of which the shape and the size of the rings changes and for instance its out-of-roundness increases. In order to make the ring suitable for use in a bearing, its dimensions should be restored within narrow tolerances, which entails a costly grinding operation.
The changes in size of the steel material can be predicted in a reasonable way based on experience. These changes result inter alia from the change in volume which occurs during the martensite transition process upon quenching. In contrast, the changes in the shape of the rings are less predictable. These changes result from xe2x80x9cinhomogenitiesxe2x80x9d in the rings, such as in their chemical composition, microstructures, possible decarburization and deformations.
However, the main variations in shape result from differences in residual stresses. Finally, the shape of the rings is influenced by plastic deformations before and during heat treatment and after quenching.
The object of the invention is therefore to provide a method by means of which the variations in shape of the rings can be controlled in a more reliable way. This object is achieved by controlling the flow supply in such a way that a desired internal stress pattern is obtained in said ring by controlling the propagation of phase transformation from austenite to martensite or bainite structure.
The flows of the cooling medium, such as a coolant spray or mist, which both are controlled accurately, are now used for establishing a stress pattern in the ring which gives rise to less distortions. In particular, for manufacturing a ring for a bearing having a raceway on one of its surfaces, the spray or mist supply to said surfaces is controlled in such a way that compression stresses are obtained in the surface zone at the raceway.
The compression stresses near the raceway, which are balanced by tensile stresses near the other surface of the ring, also have a positive influence on the behaviour of the ring under rolling contact with the rollers, in that the development of cracks which would result from rolling contact fatigue is thereby inhibited.
The method according to the invention can be further improved by clamping the ring in axial direction, the clamping force being controlled in such a way that out-of-roundness as a result of the cooling processes is minimised.
In order to control the manufacturing process accurately, the surface temperature of at least one surface is measured.
The cooling medium, such as a spray or mist, can be formed by water and air at a suitable temperature, e.g. in the rage of 0-50 degrees C and at an overpressure. Both surfaces of the rings can be handled simultaneously by a mixture of a coolant, such as water and/or air.
During this process, the ring can be rotated about its axis at an angular speed in the range of 0-200 revolutions per minute.
As mentioned, the spray or mist supply is controlled in such a way that a martensite microstructure of the steel material is obtained; at the same time, the clamping force exerted in axial direction is controlled in relation to the temperature drop of the ring to generally below the martensite transition temperature. Thus, the flows of spray or mist are controlled according to a predetermined cooling curve in the phase diagram.
The invention is also related to a steel bearing ring, manufactured by the method described before, wherein the region adjacent the surface for the raceway of the ring contains compressive stresses and the opposite region contains tensile stresses according to a desired stress pattern.
The invention is furthermore related to an apparatus for performing a heat treatment on steel rings, comprising a support means for a ring to be treated, first and second supply means for supplying a spray or mist of a liquid, gas or combination thereof to the outer surface respectively the inner surface of the ring, and control means for controlling the supply of spray or mist so as to obtain a desired internal stress pattern in the ring at ambient temperature.
Said apparatus also comprises support means having a fixture for axially clamping the ring.
The fixture comprises an upper die and a lower die, which by means of cylinder/piston devices can be moved towards each other so as to clamp a ring, which lower die is supported rotatably with respect to their axes by means of bearings.
The clamping force exerted by the cylinder/piston devices, and the rotational speed of the dies is controlled by the control means.
A short summary of the invention is given below.
The cooling process is controlled by computer with two individual operating closed loops. One loop is dedicated to controlling the temperature on the outside surface, and one loop is dedicated to control the inside surface temperature of the ring.
Before the actual cooling process is started, the required temperaturexe2x80x94time curves for the inside and outside diameter of the ring are entered into the computer as a few typical temperature-time set-points. The curves in between these set-points are calculated by the computer to obtain the two curves that completely describe the cooling curves for the inside and outside diameter of the ring. During the cooling process, the temperature of the ring is measured both on the inside and the outside diameter by means of two non-contacting temperature measuring devices. The signals of these devices are used for feedback to the controllers. The controllers on their turn adjust the water flow to the inside and outside nozzles dependant on differences between the actual measured temperatures compared to the required temperature according to the temperature set-point curves. The airflow to the nozzles can be kept at a constant level or can be made dependant of the water flow.
Since the temperature set-point curve for the inside diameter and outside diameter are completely independent of each other, it is possible to vary the cooling rate on the inside diameter independent of the outside diameter during the course of the cooling process. This is essential for being able to control the stress profile pattern through the ring cross section in a reproducible way.